Propellent powders



United States Patent PROPELLENT POWDERS Raymond H. Maag, Florissant, Mo., and Jack J. Foster,

Alton, Ill., assignors to Olin Mathieson Chemical Corporation, East Alton, 111., a corporation of Virginia No Drawing. Application November 6, 1957 Serial No. 694,708

3 Claims. (Cl. 52-20) This invention relates generally to propellent powders and particularly to a method of preparing such powders having desirable ballistic and physical characteristics.

Various types of cartridges, including shot shell cartridges, require a propellent charge having a rapid burning rate. This burning rate can be obtained by incorporating a large amount of an accelerator such as nitroglycerine into the grain and more preferably by utilizing a charge consisting partially or entirely of propellent powder grains having a porous structure. While such porous powders and various methods of manufacturing them are well known in the art, such methods are attended by numerous difficulties. For instance, porous propellent powder grains have a distinct aflinity for the absorption of waxes and oils used to treat shot shell cases and also for water. Such absorbed materials have a deleterious eiiect upon the ballistic properties of the powder. In addition, the provision of such powders requires the manufacture, storing and related handling of a considerable number of differing types of powder.

It is, therefore, an object of this invention to provide a novel process for the manufacture of powder overcoming the disadvantages of the prior art. It is also an object of this invention to provide a novel process for the modification of propellent powder grains. A more specific object of this invention is to provide a process for the manufacture of propellent powder grains having a porous interior and a dense outer layer.

In accordance with this invention, generally stated, these and other objects are accomplished by treating propellent powder grains suspended in a liquid with a solvent which is immiscible with the suspending liquid medium and adding a solute to the suspending medium while the solvent is being removed from the suspended grains. More specifically, this invention contemplates the preparation of propellent powder grains having a porous interior and a dense outer layer by suspending propellent powder grains in a non-solvent liquid medium in the presence of a protective colloid, adding a solvent for the powder grains that is only partially miscible with the suspending medium, removing a portion of the solvent from the suspended grains, adding a solute to the non-solvent medium, and then removing the remainder of the solvent from the grains.

It has been found that the addition of a solute to the non-solvent medium during the distillation period results in the formation of a propellent powder grain having a porous center and a dense exterior layer, and that the relative size of the porous central portion can be controlled by the stage of the solvent removal period at which the solute is added. Thus, by adding the solute to the suspension when only a small portion of the solvent has been removed from the grains, the resultant grains have a small porous center surrounded by a relatively thick, dense layer. Powder grains being substantially completely porous but having a relatively thin shell or layer of dense impervious material are obtained by adding the solute to the suspended powder grain after a considerable amount ice of the solvent has been removed from the grains. This invention also contemplates the further modification of the powder grains being treated by impregnating or coating them with modifiers such as nitroglycerine, dinitrotoluene and the like prior to the removal of the solvent from the suspended grains.

Ordinarily water is the most convenient non-solvent medium but other liquids which are non-solvents for the powder, such as glycerine, benzene, chlorinated hydrocarbons and the like can also be used. The amount of water or other non-solvent used must always be sufiicient to form a slurry or suspension of the powder grains in the medium without undue crowding of the suspended powder grains. Thus, in most instances, it is considered essential that the weight ratio of the non-solvent medium to powder be at least 5:1. When water is used as the suspending medium, from about 0.3% to about 1% protective colloid, such as animal glue or starch, has been found effective for the purposes of this invention.

The solvent utilized in carrying out his invention varies with the composition of the powder being treated and also with the particular non-solvent medium used. In most instances the powder contains nitrocellulose and the suspending medium is water. Thus, it is preferred to use ethyl acetate as the solvent for the propellent powder base. It will be readily understood, however, that other solvent, such as methyl acetate, isopropyl acetate, methyl isobutyl ketone, methyl ethyl ketone, and the like can also be used. In any event, it is essential that the solvent selected must be only partially miscible, i.e. between about 3% and about 25%, with the nonsolvent suspending medium and that the solvent have a boiling point less than that of the non-solvent medium so that the solvent can be readily removed from the system.

In carrying out the process of this invention, the solvent must be added to the suspension containing the powder grains at a rate to avoid local concentration of the solvent in amounts sufiicient to dissolve or distort the grains. In practice the solvent is added gradually and at a substantially constant rate over a period of time such that the solvent is incorporated into the suspension without the formation of a two-phase liquid system. The first portion of the solvent used saturates the non-solvent medium with the solvent added to the suspending medium after it has been saturated is carried by the medium directly into the powder grains without the formation of a perceptible suspension of the solvent in the suspending medium. This is an essential feature of the present invention for if large droplets of the solvent were present in the suspension they could completely fluidize a portion of the suspended grains or perhaps cause agglomeration of the treated grains.

When the powder being treated has a nitrocellulose base and ethyl acetate is used as the solvent, the weight ratio of the ethyl acetate to the powder must be maintained between about 22:1 and 1.211. Any higher ratios will result in deformation of the grains even When the agitation is maintained at the lowest level capable of supporting the grains in the non-solvent medium. Amounts of ethyl acetate lower than 1.2 parts for each part of powder will not soften the grains sutficiently for the solute to have any appreciable eifect upon the suspended powder grains. This ratio of solvent to powder will vary when other solvents are used and is dependent upon the dissolving ability of the solvent. Also other grain compositions may cause similar ratio variations.

The solute is a material that is soluble in the suspending medium but not appreciably soluble in the powder base solvent. When water is used as the suspending medium, a great variety of substances can be employed as the solute. In actual practice, sodium sulfate is selected for economic reasons but other materials, such as barium nitrate, potassium nitrate, ammonium sulfate, sugar and the like are equally suitable. When water is used as the suspending medium, the concentration of the solute, such as sodium sulfate, is maintained between about 0.6% and about 3%. Concentrations of solute above and below these limits render it diflicult to obtain uniform results.

The invention will be further clarified and more easily understood in view of the following embodiment which exemplifies practices that may be followed in carrying out the process of this invention. All proportions are expressed in parts by weight unless otherwise indicated.

About 50 parts gelatinized spherical grains of nitrocellulose base propellent powder made in accordance with the process disclosed by Schaefer in U.S. Patent 2,160,626 and having a gravimetric density of about 0.95 grams per cc. were suspended in about 450 parts of water containing about 3 parts of animal glue. These grains were substantially non-porous and did not exhibit any cross sectional density gradient. This slurry of powder in water was agitated to suspend the powder grains uniformly within the water phase and heated to a temperature of approximately 50 C. About 100 parts of ethyl acetate were then gradually added to the system over a onehour period and the agitated solution was maintained at about 50 C. for an additional three hours. At that time, about 25 parts of a solution containing approximately equal amounts of nitroglycerine and ethyl acetate were gradually added to the system over a one-hour period during which the 50 temperature was maintained. This mixture was then agitated for an additional three hours during which the temperature was elevated to about 72 C. Additional heat was then introduced into the suspension to start removal of the solvent at a constant rate by distillation at atmospheric pressure. When the temperature reached 76 C. it was maintained at this level while about 9 parts of granular sodium sulfate were added to the slurry. After all the salt had been added, the temperature of the slurry was gradually raised to a temperature of about 99 C. to effect complete removal of the solvent from the system. The total distillation required approximately 5 or 6 hours. The slurry was then allowed to cool and the powder grains were separated from the water by filtration. The resultant grains were spherical as they were originally but were now provided with a porous interior portion formed of a number of minute cavities and this porous portion was covered with an impervious external layer. These grains had a gravimetric density of about 085 gram per cc. and, in contrast to the starting material, were completely suitable for the preparation of shot shell propellent charges.

The process of this invention can be utilized to reduce the specific gravity of the interior portion of all types of commercially available propellent powder grains. Thus, the starting material can be spherical grains of gelatinized propellent powder base as disclosed by Olsen, Tibbitts and Kerone in U.S. 2,027,114 and by Schaefer in U.S. 2,160,626, as well as extruded powder grains, disc powder grains or grains of any other shape. Regardless of the particular shape of the grain entering this process, there is little or no geometric alteration of the grain.

Thus, the treated grains have substantially the same size and shape as the starting material but have a porous interior enveloped in an impervious layer of the propellent powder base. Also, the process is applicable to propellent powders having various types of bases including nitrocellulose with or without nitroglycerine, nitrostarch, polyvinyl nitrate and other gelatinizable polynitro bases.

Although the foregoing embodiment has been described in detail, many variations or modifications can be made therein by those skilled in the art without departing from the spirit and scope of this invention.

Having thus described the invention, what is claimed 1. In the preparation of propellent powder grains having a porous interior and a dense outer layer in which the relative thickness of the outer layer is predetermined from substantially non-porous grains, the process comprising suspending substantially non-porous gelatinized nitrocellulose propellent powder grains in at least 5 parts by weight of water in the presence of a protective colloid, softening the grains by adding to the suspension a nitrocellulose solvent which is between about 3% and about 25% water miscible, the amount of solvent being insufiicient to dissolve the powder grains, distilling a portion of the solvent from the grains, adding a water-soluble solute to the water and then distilling the remainder of the solvent from the grains, the area of the porous interior increasing with 'the amount of solvent removed by distillation prior to the addition of a solute to the water.

2. In the preparation of nitrocellulose propellent powder grains having a porous interior and a dense outer layer in which the relative thickness of the outer layer is predetermined, the process comprising suspending substantially non-porous grains in at least 5 parts by weight of water in the presence of a protective colloid, softening the grains by adding to the suspension a nitrocellulose solvent which is between about 3% and about 25% water miscible, the amount of solvent being insufficient to dissolve the powder grains, distilling a portion of the solvent from the grains, adding a water-soluble solute to the water and distilling the remainder of the solvent from the grain.

3. A method of controlling the relative area of internal porosity of gelatinized nitrocellulose propellent grains which comprises suspending substantially non-porous hardened grains in at least 5 parts by weight of water containing a protective colloid, adding a nitrocellulose solvent which is between about 3% and about 25% water miscible in amounts insufiicient to dissolve the grains, distilling a portion of the solvent from the grains, adding a water-soluble solute to the water, and then completing the solvent distillation, the area of internal porosity of the grains increasing as the portion of the solvent removed prior to the addition of the solute is increased.

References Cited in the file of this patent UNITED STATES PATENTS 2,160,626 Schaefer May 30, 1939 2,213,255 Olsen et al. Sept. 3, 1940 2,771,351 Holmes et a1. Nov. 20, 1956 

1. IN THE PREPARATION OF PROPELLENT POWDER GRAINS HAVING A POROUS INTERIOR AND A DENSE OUTER LAYER IN WHICH THE RELATIVE THICKNESS OF THE OUTER LAYER IS PREDETERMINED FROM SUBSTANTIALLY NON-POROUS GRAINS, THE PROCESS COMPRISING SUSPENDING SUBSTANTIALLY NON-POROUS GELATINIZED NITROCELLULOSE PROPELLENT POWDER GRAINS IN AT LEAST 5 PARTS BY WEIGHT OF WATER IN THE PRESENCE OF A PROTECTIVE COLLOID, SOFTENING THE GRAINS BY ADDING TO THE SUSPENSION A NITROCELLULOSE SOLVENT WHICH IS BETWEEN ABOUT 3% AND ABOUT 25% WATER MISCIBLE, THE AMOUNT OF SOLVENT BEING INSUFFICIENT TO DISSOLVE THE POWDER GRAINS,S DISTILLING A PORTION OF THE SOLVENT FROM THE GRAINS, ADDING A WATER-SOLUBLE SOLUTE TO THE WATER AND THEN DISTILLING THE REMAINDER OF THE SOLVENT FROM THE GRAINS, THE AREA OF THE POROUS INTERIOR INCREASING WITH THE AMOUNT OF SOLVENT REMOVED BY DISTILLATION PRIOR TO THE ADDITION OF A SOLUTE TO THE WATER. 